This allowed the following changes:
  - heap profiler now makes one pass less over the heap.
  - HeapEntriesMap does not allocate EntryInfo per each entry.
  - there's no need for an extra pass to set indexes before serialization.

As a result snapshot taking time has reduced up to 2x times.

Review URL: https://chromiumcodereview.appspot.com/10353010

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11531 ce2b1a6d-e550-0410-aec6-3dcde31c8c00